A NSW parliamentary inquiry into same-sex marriage has been told the state government doesn't have the power to redefine marriage.

The NSW Parliament's Social Issues Committee held its first public hearings in Sydney yesterday. The Committee heard evidence from eminent constitutional and family law experts from across Australia.

University of Sydney law professor Patrick Parkinson told the inquiry the state government, "could not deliver what the [same-sex marriage] advocates want it to deliver."

Witnesses at yesterday's hearing described the serious and significant legal impediments preventing states from trying to go-it-alone on marriage laws.

A number of witnesses referred to a 2004 amendment to the federal Marriage Act which defined marriage as the union of a man and a woman. They argued the federal parliament had sought to "cover the field" and there was no room for states to redefine marriage.

Last year the Tasmanian parliament rejected a bill that sought to redefine marriage in that state. South Australia is also considering similar legislation.

The NSW inquiry will hold its second and final public hearing next week and is expected to present its final report by July 2013.